Superconductivity was originally discovered by the Dutch scientist Heike Onnes in 1911 while he was studying the electrical properties of mercury at very low temperatures. In more recent times, Ogg (1946) studied superconductivity in ammonia solutions and proposed that superconductivity arose in these quenched metal-ammonia solutions because of mobile electron pairs. About 1973, it was determined that certain niobium metal alloys exhibited superconductivity when cooled to liquid helium (4.degree. K.) temperatures. Later results in the 1970's raised this temperature as high as 23.degree. K. (-250.degree. C.). Until recently, it was believed that superconductivity above this temperature was not possible. This belief was based on the theoretical work of Bardeen, Cooper and Schieffer (BCS theory-1946) which predicted such a limit. In December 1986, Bednorz and Muller announced the discovery (G. Bednorz and A. Mller, Z. Phys., B64 189 (1986)) of a new ceramic superconducting compound based on lanthanum, barium, and copper oxides, whose critical temperature for superconductivity was close to 35.degree. K. By the following month, the critical temperature, Tc, for the onset of superconductivity was raised to nearly 80.degree. K. by C. W. Chu and coworkers (M. K. Wu, J. R. Ashburn, C. J. Tang, P. H. Hor, R. L. Meng, L. Gao, Z. J. Huang, Y. Q. Wang and C. W. Chu, Phys. Rev. Lett. 58 908 (1987)). This was achieved by changing the composition to yttrium barium copper oxide, approximated by the formula: Y.sub.1.0 Ba.sub.1.8 Cu.sub.3.0 O.sub.6.3. Since then, a number of families of superconducting ceramic oxides have been investigated, including:
Bismuth Strontium Calcium Copper Oxide: PA0 Thallium Calcium (Barium) Copper Oxide:
Bi.sub.2 Sr.sub.3-x Ca.sub.x Cu.sub.2 O 8+y2 PA1 T.sub.c =114.degree. K. PA1 Tl Ba.sub.2 Ca Cu.sub.2 O.sub.7 PA1 Tl Ba.sub.2 Ca.sub.2 Cu.sub.3 O.sub.9 PA1 Tl Ba.sub.2 Ca.sub.3 Cu.sub.4 O.sub.11 PA1 Tl Ba.sub.2 Ca.sub.4 Cu.sub.5 O.sub.13 PA1 Tc=120.degree. K. PA1 BaO--K.sub.2 O--Bi.sub.2 O.sub.3 PA1 Tc=30 K. PA1 (a) positioning a high-critical-temperature superconductive quantum interference device having a pair of insulated junctions in a superconductor loop in flux-sensing relationship to a location adjoining said body; PA1 (b) relatively displacing said body and said location; PA1 (c) while said body and said location are relatively displaced, exciting said body with a modulated alternating current and detecting with said device flux irregularities representing flaws in said body; and PA1 (d) establishing locations of said flaws by the relative positions of said location and said body upon the detection of said flux irregularities.
There have been some scattered reports of superconductivity above 162 K., For instance, R. G. Kulkarui has reported superconducting oxides having an approximate composition 0.5 CaO. 0.5 ZnO. Fe.sub.2 O.sub.4, with critical temperatures in this range. Ogushi also reported superconductivity at room temperature in yet ill-defined niobium strontium lanthanum oxides. While these reports have yet to be confirmed independently by other researchers, it is reasonable to expect that superconductors with critical temperatures near to room temperature will soon be obtained.
Niobium-based superconducting alloy wires have long been used for detecting small changes in magnetic field strength. In the prior art, the non-destructive evaluation of defects in metals employs an eddy current technique wherein the impedance change of the primary exciting coil is measured and correlated to the presence of defects and flaws within the metal sample. The method is cumbersome and must be carried out at liquid helium temperatures (.about.4 K.), which is expensive. In this method of nondestructive evaluation, currents are caused to flow in a test specimen by placing it within, or in close proximity to, the primary magnetic field of a probe coil (or array of coils). In turn, these induced currents generate a secondary magnetic field, which, by Lenz's law, opposes the primary magnetic field, thus affecting the impedance of the probe or exciting coil. By using Green's function technique and the Born Approximation, one can demonstrate that the impedance of a single turn coil, which surrounds a conducting non-magnetic cylinder containing a flaw and/or defect, will depend on the size of said flaw, its conductivity, and its depth.
The change of impedance, DZ, due to the flaw is proportional to the change of conductivity D.sigma.=.sigma.-.sigma..sub.o and the square of the flaw length. The flaw depth affects the impedance change differently, depending upon the ratio of flaw position r', to the skin depth. In certain situations, some aspects of the geometry of the flaw can be determined by measuring the probe coil impedance as the probe coil is scanned across the surface containing the flaw.
I have determined that the use of superconductor quantum interference devices to measure flaws and defects in metals produces results much superior to any known heretofore, especially when said devices are used in conjunction with modulated high frequency exciting currents. This is accomplished through exciting coils placed around the metal plate, or wrapped around a cylinder. The arrangement of the SQUIDs includes an alternating bias current, and flux modulation in a locked flux-loop design. This mode of operation ensures minimum noise of voltages produced by the superconductor quantum interference devices, which may result because of critical current fluctuations in the non-identical junctions of the superconducting interference devices, and also because of fluctuations in junction resistances. I have further established that my new and novel methods for non-destructive detection of flaws in metals can be applied to both non-magnetic and ferric-magnetic materials, in contrast to the prior art. I have further determined that the technique of the instant invention is very sensitive, compared to those of the prior art.